+ Post New Thread
Results 1 to 16 of 16
  1. #1
    Newbie level 4
    Points: 40, Level: 1

    Join Date
    Feb 2019
    Posts
    7
    Helped
    0 / 0
    Points
    40
    Level
    1

    RF Detector not detecting cellular phones

    Hi all,

    I was hoping somebody can tell me why the below rf detector is not picking up any cellular phone signals? I am using through hole components and have soldered everything onto a CNC made PCB boards, keeping all tracks as short as possible (see image). I have no problem detecting 27Mhz, 40Mhz (wireless home phone), 466Mhz (walkie-talkie) but I am getting nothing from 2.4Ghz WiFi or 3G/4G frequency(s) even when holding the phone literally a few millimeters next to the antenna. I have ordered a high frequency, wideband LNA monolithic amplifier to try boosting the signal before rectification eventhough I think a cell phone transmitting at at least 1 watt when attempting to make a connection immediately next to the pickup antenna would not need a LNA pre-amp stage. Curious to hear some of you more experienced RF guys input!!!


    Click image for larger version. 

Name:	rf_detector.jpg 
Views:	29 
Size:	49.7 KB 
ID:	151029

    Click image for larger version. 

Name:	rf_detector_1e.jpg 
Views:	18 
Size:	104.5 KB 
ID:	151031

  2. #2
    Advanced Member level 5
    Points: 22,507, Level: 36
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    4,301
    Helped
    954 / 954
    Points
    22,507
    Level
    36

    Re: RF Detector not detecting cellular phones

    I don't think that diode is appropriate for 2GHz.

    After a closer look, maybe it's ok. But I would look carefully at your layout.
    Last edited by barry; 8th February 2019 at 17:38.



  3. #3
    Advanced Member level 5
    Points: 28,267, Level: 41
    BigBoss's Avatar
    Join Date
    Nov 2001
    Location
    Turkey
    Posts
    4,077
    Helped
    1233 / 1233
    Points
    28,267
    Level
    41

    Re: RF Detector not detecting cellular phones

    I guess the diode may work up to 1GHz but a signal amplifier ( even a simple one with a single transistor) will help to detect the signal strength.
    The diode is not appropriate at all for 2.4GHz..



    •   AltAdvertisment

        
       

  4. #4
    Newbie level 4
    Points: 40, Level: 1

    Join Date
    Feb 2019
    Posts
    7
    Helped
    0 / 0
    Points
    40
    Level
    1

    Re: RF Detector not detecting cellular phones

    Thanks for the response guys!

    Why would the diode be inappropriate for 2Ghz? It has 2pF junction capacitance. Plus, others seem to be using it successfully for fine tuning their 2.4Ghz RC transmitters. See link.

    https://www.rcgroups.com/forums/show...h-meter-for-rc

    4G carrier frequencies in my country are anywhere between 850Mhz and 2.6Ghz so I am thinking I should be picking up something. I think I should mention that I have also tried a pair of BAT62 diodes but on the breadboard circuit, not PCB and that also had no issues detecting 40Mhz and 466Mhz but not the cell phone.
    Last edited by M84AB3; 9th February 2019 at 05:14.



    •   AltAdvertisment

        
       

  5. #5
    Advanced Member level 5
    Points: 34,039, Level: 45

    Join Date
    Dec 2004
    Location
    New England, USA
    Posts
    4,789
    Helped
    1336 / 1336
    Points
    34,039
    Level
    45

    Re: RF Detector not detecting cellular phones

    "It has 2pF junction capacitance."

    You do realize that 2 pf is a HUGE amount of capacitance at 2.4 GHz, right?
    Get a diode with 0.2 pF.

    Also, does your PC board have a ground plane on the backside?

    I would also change it to a single diode.



    •   AltAdvertisment

        
       

  6. #6
    Advanced Member level 5
    Points: 33,423, Level: 44

    Join Date
    Oct 2001
    Posts
    4,779
    Helped
    1412 / 1412
    Points
    33,423
    Level
    44

    Re: RF Detector not detecting cellular phones

    On the net I found another schematic, also using 1N5711 diode, but followed by an amplifier.
    http://www.techlib.com/electronics/cellhelp.htm
    This schematic was designed by Charles Wenzel, one of the biggest guru in RF.

    Using the spice model offered by ST for 1N5711 and using LTspice, I see that 1N5711 works fine at 2.4GHz AS a detector.

    Remove the 50 ohms resistor at the input of your detector, and probably your circuit will work just fine.



  7. #7
    Newbie level 4
    Points: 40, Level: 1

    Join Date
    Feb 2019
    Posts
    7
    Helped
    0 / 0
    Points
    40
    Level
    1

    Re: RF Detector not detecting cellular phones

    "You do realize that 2 pf is a HUGE amount of capacitance at 2.4 GHz, right?"
    Obviously not big enough to stop a 2.4Ghz FlySky FS-HF030 RC transmitter. See video below. I do mention in the same post that I have tried a pair of BAT62 diodes that are advertised, in the datasheet, as "Low barrier diode for detectors up to GHz frequencies" and have capacitance of 0.35pF (typical) but that too produced no results when trying to detect my cell phone (iphone 6s on 4G network). Neither did it work in detecting the 2.4Ghz WiFi signal from my router.

    I had forgotten that I had a 2.4Ghz RC transmitter and tried that and sure enough the rf gets detected. It is based on the A7105 chip and uses FSK/GFSK modulation. This to me at least proves that the circuit works at least up to the 2-3Ghz band, with the highest currently operational 4G frequency being 2.6Ghz, only 200Mhz above 2.4Ghz. I thus cant help but wonder if is something to do with the short cell phone bursts? All three devices I demonstrate in the video below have a continuously emitted RF signal but I am not sure if cell phones transmit the same way.



    - - - Updated - - -

    "Also, does your PC board have a ground plane on the backside??"
    I would say so, see attached image.

    Click image for larger version. 

Name:	WP_20140522_007a.jpg 
Views:	11 
Size:	257.5 KB 
ID:	151061



  8. #8
    Super Moderator
    Points: 251,373, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    43,773
    Helped
    13308 / 13308
    Points
    251,373
    Level
    100

    Re: RF Detector not detecting cellular phones

    I would say so, see attached image.
    Unfortunately the ground plane is unconnected.



  9. #9
    Advanced Member level 5
    Points: 22,507, Level: 36
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    4,301
    Helped
    954 / 954
    Points
    22,507
    Level
    36

    Re: RF Detector not detecting cellular phones

    Quote Originally Posted by FvM View Post
    Unfortunately the ground plane is unconnected.
    Lol.

    And 2pf at 2.4 GHz is not HUGE, unless you consider 30 ohms HUGE.



  10. #10
    Advanced Member level 5
    Points: 33,423, Level: 44

    Join Date
    Oct 2001
    Posts
    4,779
    Helped
    1412 / 1412
    Points
    33,423
    Level
    44

    Re: RF Detector not detecting cellular phones

    Quote Originally Posted by M84AB3 View Post
    All three devices I demonstrate in the video below have a continuously emitted RF signal but I am not sure if cell phones transmit the same way.
    As I mentioned, seems that the detector works at 2.4GHz. I see that you didn't remove the 50 ohms at the input. That will improve a bit the sensitivity of the detector. You can replace that resistor with a 0.1uH RF choke (the same as in the 2nd schematic in link that I posted above).

    Cell phones in 2G mode transmit bursts with duty cycles varying between 1/8 and 4/8. In 3G and 4G modes there is a continuous transmission during a call.
    In cell phones, their output power vary from -50dBm to about +24dBm in 3G and 4G and up to +33dBm in 2G. So the power is not constant during a call as in your cordless phone or RC transmitter.



  11. #11
    Advanced Member level 5
    Points: 12,117, Level: 26

    Join Date
    Apr 2014
    Posts
    1,890
    Helped
    758 / 758
    Points
    12,117
    Level
    26

    Re: RF Detector not detecting cellular phones

    Quote Originally Posted by vfone View Post
    As I mentioned, seems that the detector works at 2.4GHz.
    But this layout is far from ideal. By shrinking the layout and using proper RF layout technique, the detector could be much more sensitive.



    •   AltAdvertisment

        
       

  12. #12
    Advanced Member level 5
    Points: 33,423, Level: 44

    Join Date
    Oct 2001
    Posts
    4,779
    Helped
    1412 / 1412
    Points
    33,423
    Level
    44

    Re: RF Detector not detecting cellular phones

    Totally agree, a good PCB would help. And is not impossible because seems that have access to a PCB milling machine, which makes the job easier.
    But even the best PCB design would not help if a relative high impedance detector input is shortened to the ground with a 50 ohms resistor.



  13. #13
    Newbie level 4
    Points: 40, Level: 1

    Join Date
    Feb 2019
    Posts
    7
    Helped
    0 / 0
    Points
    40
    Level
    1

    Re: RF Detector not detecting cellular phones

    Quote Originally Posted by vfone View Post
    As I mentioned, seems that the detector works at 2.4GHz. I see that you didn't remove the 50 ohms at the input. That will improve a bit the sensitivity of the detector. You can replace that resistor with a 0.1uH RF choke (the same as in the 2nd schematic in link that I posted above).

    Cell phones in 2G mode transmit bursts with duty cycles varying between 1/8 and 4/8. In 3G and 4G modes there is a continuous transmission during a call.
    In cell phones, their output power vary from -50dBm to about +24dBm in 3G and 4G and up to +33dBm in 2G. So the power is not constant during a call as in your cordless phone or RC transmitter.
    I did remove the 50 ohm resistor by cutting the bottom lead with wire cutters. It is not obvious in the video. If the 4G transmission is continuous then why am I not able to pick it up with the antennas literally touching one another? If it works for 2.4Ghz RC transmitter, I don't see why it wouldn't work for a cell phone.

    Quote Originally Posted by volker@muehlhaus View Post
    But this layout is far from ideal. By shrinking the layout and using proper RF layout technique, the detector could be much more sensitive.
    As I do have a CNC machine I will go all out SMD on this one and see what results I get. I have ordered a few MGA-86563 MMICs to use as the front-end LNA.
    Last edited by M84AB3; 11th February 2019 at 15:42.



  14. #14
    Super Moderator
    Points: 69,422, Level: 64
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    14,177
    Helped
    3237 / 3237
    Points
    69,422
    Level
    64

    Re: RF Detector not detecting cellular phones

    Hi,

    As I do have a CNC machine I will go all out SMD on this one and see what results I get.
    I see the "ground plane" problem much more important than the SMD/THM problem.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  15. #15
    Newbie level 4
    Points: 40, Level: 1

    Join Date
    Feb 2019
    Posts
    7
    Helped
    0 / 0
    Points
    40
    Level
    1

    Re: RF Detector not detecting cellular phones

    Quote Originally Posted by KlausST View Post
    Hi,


    I see the "ground plane" problem much more important than the SMD/THM problem.

    Klaus
    Can you please explain in a little more detail? That statement alone doesn't tell me much.

    By ground plane I assume you mean something as described here under the "Microstrip" section.



  16. #16
    Super Moderator
    Points: 69,422, Level: 64
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    14,177
    Helped
    3237 / 3237
    Points
    69,422
    Level
    64

    Re: RF Detector not detecting cellular phones

    Hi,

    as stated in post#8:
    It doesn´t help to generate a large area of "unconnected" copper. You need to connect it to system_GND and need to use it as GND_Plane.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



--[[ ]]--